When the Terminator, the teen John Connor, Sarah Connor and Miles Dyson go to the Cyberdyne Systems Corporation building in order to destroy any data that could lead to Skynet's creation (and therefore to Judgment Day), the alarm is raised by the reception guards and as a consequence all entry panels are deactivated.

At this point Miles Dyson's knowledge is useless. He knows the codes but everything is remotely blocked. John Connor uses his "Pin cracker" device, used for cracking ATMs, and tries to get the code and so open the safe.

But here's the problem... Myles Dyson already knew the codes. There was no need to "get them" illegally. The problem is not there, the problem is that the panels are blocked anyways, codes are irrelevant. Not to mention that an ATM probably works very differently from a security panel in a secret research building.

2 Answers
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It neutralises the user codes in the building, there would still be master codes etc. Otherwise how would the police move around in the building, what if there was a fire etc.

It is this master code that John cracked.

As for the ATM working differerntly: What John appeared to have was a brute force number generator (perhaps with a little optimising AI), therefore any machine which relied on combination entry would be vulnerable to it. The machine was not designed to purely work on ATMs, that is merely the main thing John used it for.

Uhm interesting but that's speculation, right? There is no mention of master code in the movie, as far as I know.
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AlenannoAug 23 '12 at 11:38

@Alenanno - it is speculative and I'm sure there is no mention of override codes, but its not unreasonable to think that there might be code(s) that would not be disabled. It is more of a plot hole that an ATM cracking device would be usable in this situation at all.
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iandotkelly♦Aug 23 '12 at 18:51

@Alenanno - Yes, it's speculation on my part. Regarding the ATM: the third script excerpt says: which is running code combinations, suggesting a brute force method. I will rework my answer to make it more clear.
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Oliver_CAug 24 '12 at 7:21